This invention relates to a fuel injector for use in supplying fuel, under pressure, to the cylinders of an internal combustion engine.
A known fuel injector arrangement comprises a plunger reciprocable within a plunger bore provided in a housing to pressurise fuel located within a pump chamber defined by the plunger bore. The plunger bore communicates with a fuel pressure actuated injector such that, once the fuel pressure within the plunger bore exceeds a predetermined level, the injector opens and, thus, fuel injection commences.
In order to permit independent control of the injection pressure and the timing of injection, it is known to provide a spill valve which communicates with the pump chamber, and an injection control valve which controls fuel pressure within a control chamber defined, in part, by a surface associated with a valve needle of the injector to control movement of the needle. In use, the spill valve remains open during initial inward movement of the plunger. Subsequently, the spill valve is closed, further inward movement of the plunger pressurising the fuel within the pump chamber. When injection is to commence, the injection control valve is actuated to connect the control chamber to a low pressure drain thus permitting movement of the needle away from its seating to commence fuel injection.
European Patent Application No 0840003 describes a fuel injector of the aforementioned type, including a spill valve arrangement, which is controlled by means of a first actuator, and an injection control valve, which is controlled by means of a second actuator. As the fuel injector requires two actuator arrangements to independently control the spill valve and the injection control valve, the cost of the injector is high, particularly if piezoelectric actuators are employed.
European Patent Application No 0905719 describes a method of independently controlling the spill valve and the injection control valve of a fuel injector by means of a single, electromagnetic actuator arrangement. When it is desired to pressurise fuel within the pump chamber, the spill valve is closed by energising the actuator to a first, relatively low energisation level, during which stage of operation the injection control valve remains closed to low pressure. In order to commence fuel injection, the actuator is energised to a second, higher energisation level to open the injection control valve to low pressure. When the injection control valve is open to low pressure, the fuel injector valve needle is moved inwardly to permit fuel injection through outlet openings provided in the fuel injector nozzle body.
In order to permit full filling of the pump chamber and to minimise the volume of the plunger bore and maximise the peak fuel pressure, it is desirable to locate the spill valve in close proximity to the plunger. In addition, it is desirable to locate the injection control valve in close proximity to the valve needle as this improves the response of the valve needle. However, it is difficult to satisfy both of these requirements using the fuel injector arrangements described hereinbefore.
It is an object of the invention to provide a fuel injector which alleviates this problem.
According to the present invention there is provided a fuel injector for use in an injector arrangement including a pumping plunger and a pump chamber and a spill valve arrangement controlling communication between the pump chamber and a low pressure fuel reservoir, the fuel injector comprising a valve needle engageable with a seating to control fuel delivery through a fuel injector outlet opening, a first control chamber being arranged such that the fuel pressure therein acts on a surface associated with the valve needle and a second control chamber being arranged such that fuel pressure therein acts on a surface associated with the spill valve arrangement, and a piezoelectric actuator arrangement including a piezoelectric element having first and second ends, the first end being associated with the spill valve arrangement and the second end being associated with the valve needle such that, in use, operation of the spill valve arrangement and movement of the valve needle is controlled by controlling the energisation level of the piezoelectric element.
The invention provides the advantage that only a single piezoelectric actuator arrangement is required. This reduces the cost of the fuel injector. Additionally, the spill valve and the control chamber associated with the valve needle can both be located in close proximity to the piezoelectric actuator arrangement. Thus, the response of the valve needle is improved.
The fuel injector conveniently includes first and second piston members, the first piston member being moveable with the first end of the piezoelectric element and the second piston member being movable with the second end of the piezoelectric element, movement of the first and second piston members being controlled by varying the energisation level of the piezoelectric element.
Conveniently, the first and second piston members are slidable within first and second bores respectively. The first bore may define, in part, the first control chamber and the second bore may define, in part, the second control chamber. Movement of the first piston member within the first bore varies the volume of the first control chamber and movement of the second piston member within the second bore varies the volume of the second control chamber.
The first piston member may have a smaller diameter than the second piston member such that de-energisation of the piezoelectric element to a first energisation level causes movement of the second piston member to close the spill valve arrangement and further de-energisation of the piezoelectric element to a second, lower energisation level causes movement of the first piston member to move the valve needle away from the seating.
Alternatively, the first piston member may have a diameter greater than the diameter of the second piston member such that energisation of the piezoelectric element to a first energisation level causes movement of the second piston member to close the spill valve arrangement and further energisation of the piezoelectric element to a second, higher energisation level causes movement of the first piston member to move the valve needle away from the seating.
In one embodiment of the invention, the fuel injector may further comprise an injection control valve arrangement including a control valve member and a third control chamber for fuel, a surface of the control valve member being exposed to fuel pressure within the third control chamber, the control valve member being engageable with a further seating to control fuel pressure within the first control chamber.
Conveniently, the spill valve arrangement comprises a spill valve member which is slidable within a further bore defining, in part, an additional chamber. The spill valve member may be engageable with a further seating to control communication between the additional chamber and a low pressure fuel reservoir. Alternatively, the additional chamber may communicate directly with a low pressure fuel reservoir.
In an alternative embodiment, the first control chamber may be arranged to communicate directly, by means of a first restricted flow path, with a supply passage for supplying fuel under high pressure to the injector.
The second control chamber may be arranged to communicate directly, by means of a second restricted flow path, with a supply passage for supplying fuel under high pressure to the injector.
The piezoelectric actuator arrangement may include a single piezoelectric element or may include a stack of piezoelectric elements.
The valve needle may be of the inwardly or outwardly opening type.